1. Field of the Invention
The present invention is related to an ion beam control apparatus and its control method capable of controlling ion beams.
2. Description of the Related Art
FIG. 12 is a diagram showing the configuration of a conventional ion beam control apparatus. FIG. 13 is a diagram showing the intensity distribution in relation to the ion beam energy. FIG. 13(A) is a view showing a frame format of the intensity distribution in relation to the energy of ion beams generated based on the irradiation of a pulse laser, FIG. 13(B) is a view showing a frame format of the intensity distribution in relation to the energy of ion beams controlled with the ion beam control apparatus, and FIG. 13(C) is a diagram showing the test results thereof. The horizontal axis of FIG. 13(A) to FIG. 13(C) shows the energy, and the vertical axis thereof shows the ion beam intensity; that is, the existing probability of ions. The unit of the horizontal axis of FIG. 13(C) is MeV, and the unit of the vertical axis thereof is 109/sr/shot/100 keV.
As shown in FIG. 12, when a pulsed laser beam (pulse laser beam) PL is irradiated onto a solid thin film MF, the solid thin film MF will be heated by the pulse laser beam PL, and an ion beam IB is created thereby. With respect to the intensity distribution (energy spectrum) in relation to the energy of the ion beam IB, as shown in FIG. 13(A), in parallel with a peak formation of the intensity (existing probability of ions) on the low energy side, the intensity will exponentially decrease together with the increase of energy. Thus, if the user wishes to use an ion beam IB having a prescribed energy, there are cases where the prescribed intensity cannot be obtained, and there is an inconvenience upon using the ion beam IB.
Thus, proposed is a method of using the ion beam control apparatus 1000 shown in FIG. 12 to control the intensity distribution in relation to the energy of the ion beam IB using the phase rotation by a radio frequency electric field (A. Noda et al., Beam Science of Technology, Vol. 6, 2001. PP 21-23 and A. Noda et al., Laser Physics 2006, Vol. 16, No. 4, PP 647-653). The ion beam control apparatus 1000 shown in FIG. 12 comprises a cylindrical case 1001, and a radio frequency electric field generation unit 1002 (1002-1 to 1002-3) for generating a radio frequency electric field E(t). The radio frequency electric field generation unit 1002 comprises three cylindrical first to third electric field generation components 1002-1 to 1002-3 arranged so the ion beam IB passes therethrough and the radio frequency electric field E(t) affects the ion beam IB. The first to third electric field generation components 1002-1 to 1002-3 are respectively arranged continuously from one side face to the other side face of the case 1001 so that they will mutually have prescribed spacings, so that the central axes will mutually coincide, and so that the central axis of the first to third electric field components 1002-1 to 1002-3 and the central axis of the case 1001 will mutually become orthogonal. More specifically, the first electric field generation component 1002-1 is disposed on one side face of the case 1001 so as to penetrate the side face of the case 1001, and so that the central axis of the first electric field generation component 1002-1 and the central axis of the case 1001 will mutually become orthogonal. The second electric field generation component 1002-2 is disposed in the case 1001 by being supported by a support member 1003 that is suspended from the inner face of the top cover of the case 1001 at a prescribed spacing from the first electric field generation component 1002-1 and so that the central axis of the first electric field generation component 1002-1 and the central axis of the second electric field generation component 1002-2 will mutually coincide. The third electric field generation component 1002-3 is disposed on the other side face facing the one side face of the case 1001 at a prescribed spacing from the second electric field generation component 1002-2 and so that the central axis of the second electric field generation component 1002-2 and the central axis of the third electric field generation component 1002-3 will mutually coincide, and so as to penetrate the side face of the case 1001. A radio frequency (RF) AC voltage is applied to the case 1001 and the radio frequency electric field generation unit 1002 with an AC source not shown. Consequently, a radio frequency electric field E(t) will be generated in the first space (first gap) G1 between the first electric field generation component 1002-1 and the second electric field generation component 1002-2, and the second space (second gap) G2 between the second electric field generation component 1002-2 and the third electric field generation member 1002-3, respectively.
With the ion beam control apparatus 1000 configured as described above, a peak PK of the intensity can be generated in a given energy of the ion beam IB using the phase rotation by the radio frequency electric field E(t) as shown in FIG. 13(B). This has been confirmed based on testing as shown in FIG. 13(C). Δ of FIG. 13(C) shows the test results in a case without phase rotation, □ shows the test results in a case where the phase rotation is applied with a relative phase between the pulse laser and an RF electric field of 80 degrees, and ▪ shows the test results in a case where the phase rotation is applied with a relative phase between the pulse laser and an RF electric field of 215 degrees. In the case without phase rotation, together with the increase of energy, the intensity is decreasing gradually. However, in cases with phase rotation of 80 degrees or 215 degrees, plural intensity peaks are formed.
This can be explained as follows. FIG. 14 is a diagram explaining the phase rotation of ion beams. The horizontal axis of FIG. 14 shows the phase of the radio frequency electric field, and the vertical axis thereof shows the strength of the radio frequency electric field E(t).
The ion beam IB generated with the pulse laser beam PL is configured, as shown in FIG. 13(A), plural ions having various values of energy, and high energy ions will reach the first gap G1 faster than the low energy ions. Thus, as shown in FIG. 14, with the ion Pa having the reference energy to form an intensity peak, the ion Pb having an energy that is higher than the ion Pa will reach the first gap G1 faster than the ion Pa, and the ion Pc having an energy that is lower than the ion Pa will reach the first gap G1 later than the ion Pa. Thus, if the radio frequency electric field E(t) is caused to affect the ion beam so as to accelerate the ion Pb having an energy that is higher than the ion Pa in the amount of energy that is smaller than the ion Pa, and accelerate the ion Pc having an energy that is lower than the ion Pa in the amount of energy that is greater than the ion Pa, and the distance L between the first gap G1 and the second gap G2 is set so that the radio frequency electric field E(t) affects the ion Pa with the same phase in the first gap G1 and the second gap G2, the energy of the ion Pb and the energy of the ion Pc will respectively approach the energy of the ion Pa. Consequently, the intensity peak PK can be formed at the energy of the ion Pa. The compression of the energy expansion of the ion beam IB using the radio frequency electric field E(t) is referred to as the phase rotation by the radio frequency electric field.
On the other hand, as an application example of this kind of ion beam IB, for instance, there are the surface treatment technology of performing surface modification by injecting trace amounts of elements as a result of irradiating the ion beam IB, and the particle radiation therapy that causes damage to the cancer cells by irradiating the ion beam IB. Generally speaking, with the radiation of X rays, γ rays and so on and neutron radiation, the dose will become maximum at a site that is relatively shallow in the body, and the dose will gradually decrease. Thus, with radiation of X rays, γ rays and so on and neutron radiation, damage will also be inflicted on the normal cells around the cancer. Meanwhile, with a particle beam such as a proton beam or a carbon ion beam, the dose will be relatively low on the body surface, and becomes a maximum dose (Bragg peak) at the deepest site, and the dose will be nearly zero at any deeper site. Thus, with the particle beam, if the Bragg peak is irradiated to match the affected area of the cancer, it is possible to damage the cancer cells without much affecting the normal cells around the depth direction. Conventionally, in order to perform this kind of particle radiation therapy, a relatively large accelerator such as a cyclotron or a synchrotron was required.
With the foregoing ion beam control apparatus 1000, although it is possible to control the formation of the intensity peak PK to a prescribed energy, the ion beam IB will diverge in the moving radius direction (perpendicular direction in relation to the traveling direction). Moreover, since an electrostatic breakdown will occur if high voltage of a certain level or higher is applied to the first to third electric field generation components 1002-1 to 1002-3, the voltage that can be applied to the first to third electric field generation components 1002-1 to 1002-3 will be limited. Consequently, the phase rotation of the high energy ion beam IB cannot be performed.
In particular, when considering the use of the ion beam IB, it is desirable to control (convergence in the moving radius direction) of the energy expansion level in the ion beam IB and the energy expansion level in the moving radius direction, and the realization of high energy.